Apparatus for optically monitoring the gyromagnetic resonance of quantum systems

ABSTRACT

There is disclosed improved magnetometer and frequency stabilizing apparatus which utilizes the principles of optical alignment and monitoring of quantum systems. Optical irradiation of said quantum systems in a unidirectional magnetic field effects alignment which alignment may then be monitored by detecting the nonabsorbed pumping radiation. Realignment of the quantum system by application of a radio frequency magnetic field results in changes in the detected nonabsorbed radiation which changes may be used to control either the frequency of the radio frequency magnetic field or the intensity of said unidirectional magnetic field to maintain said quantum systems at resonance. Quantum systems of the type disclosed include the alkali atoms, such as potassium, rubidium, sodium and cesium which are quantum systems exhibiting nonvanishing total angular momentum.

United States Patent 2,884,524 4/1959 Dicke Appl. No. Filed PatentedAssignee APPARATUS FOR OP'l'lCALLY MONITORING THE GYROMAGNETIC RESONANCEOF QUANTUM SYSTEMS 9 Claims, 6 Drawing Figs.

US. Cl. 324/05,

Int. Cl. G0ln 33/08- Field of Search 324/05, (Inquired); 331/945, 94, 3

References Cited UNITED STATES PATENTS Primary Examiner-Rudolph V.Rolinec Assistant ExaminerMichael J. Lynch Att0rneyStanley Z. ColeABSTRACT: There is disclosed improved magnetometer and frequencystabilizing apparatus which utilizes the principles of optical alignmentand monitoring of quantum systems. Optical irradiation of said quantumsystems in a unidirectional magnetic field effects alignment whichalignment may then be monitored by detecting the nonabsorbed pumpingradiation. Realignment of the quantum system by application of a radiofrequency magnetic field results in changes in the detected nonabsorbedradiation which changes may be used to control either the frequency ofthe radio frequency magnetic field or the intensity of saidunidirectional magnetic field to maintain said quantum systems atresonance. Quantum systems of the type disclosed include the'alkaliatoms, such as potassium, rubidium, sodium and cesium which are quantumsystems exhibiting nonvanishing total angular momentum;

IQ- AMPLIFIER GENERATOR TRANSMITTED LIGHT INTENSITY P'ATENTEU mom$575,555

SHEET 2 0F 3 I772 TRANSITIONS B MC. =o AM=il EFFECTIVELY FIELDINDEPENDENT FIG.4

AMPLIFIER GENERATOR FREQUENCY TUNER PHASE SENSITIVE DETECTOR SWEEPGENERATOR FREQUENCY MEASURING CIRCUIT INVENTOR.

HANS G. DEHMELT A ORNEY PATENIEUIPMOIIII 3575.655

SHEET 3 0F 3 Io 1 l6 I8 I! f L AMPLIFIER I II 30 37 RF GENERATOR T 27 IPHASE FREQUENCY J MODULATOR TUNER 1 T MODULATION PHASE OSCILLATORSENS'T'VE DETECTOR L V ,za @RECORDER 33 UTILIZATION CIRCUIT INVENTOR.HAN s. DEHMELT BYv A A ORNE'Y APPARATUS Milli UWMIALLY MGNHTOIRIWG'llllllE GlfikflllvilAGhllE'll'lC WESUNANCE OF QUANTUM SYSTEMS Thisapplication is a continuation-in-part of copending application Ser. No.350,887, filed Mar. 10, 1964, now abandoned, which is a continuation ofabandoned application Scr. No. 649,191, filed Mar. 28, 1957.

This invention is related to the inventions disclosed and claimed inU.S. Pat. No. 3,071,721, entitled Optical Absorption Monitoring ofOriented or Aligned Quantum Systems, and patent application Ser. No.649,190, entitled Optical Absorption Monitoring of Aligned AlkaliAtoms", filed Mar. 28, 1957, abandoned in favor of continuationapplication Ser. No. 313,186, U.S. Pat. No. 3,267,360 each applicationfiled in behalf of the subject applicant and assigned to the sameassignee. in particular, the latter patent application discloses a novelmethod and apparatus for producing alignment of one optical electronquantum systems (the optical energy level structures of which areattributable to a single unpaired electron in an outermost S or Pshell), such as the atoms of the alkali group of elements, with longrelaxation times in an external magnetic field by optical pumpingtechniques and the subsequent optical detection of the light utilizedfor the optical pumping whereby the alignment of the atoms is monitored.The present invention has for its purpose the production and opticaldetection of paramagnetic resonance of quantum systems with extremelynarrow line widths and very high signal-to-noise ratios, such as thedescribed alkali atoms including potassium, rubidium and cesium whichare quantum systems exhibiting nonvanishing total angular momentum.

ln patent application Ser. No. 313,186, sodium atoms are employed, byway of example, to describe the invention and this procedure will befollowed in this application for continuity. As set forth in this patentapplication, sodium atoms in vapor form are mixed in an absorptionvessel with a buffer gas of argon which greatly increases the relaxationtime of the sodium atoms. The vessel is located in a unidirectionalmagnetic field H which may, for example, be the earths magnetic field. Asodium light source producing A5896 radiation is provided, thisradiation being focused in a beam parallel to the unidirectionalmagnetic field through a circular polarizer and through the absorptionvessel, the beam, after passing through the vessel, being focused on aradiationresponsive apparatus such as a photocell. This circularlypolarized light produces a substantial alignment of the sodium atoms inthe unidirectional magnetic field by the process of optical pumping,that is, an overpopulation of certain of the nonabsorbing energysublevels of the ground state of the sodium atoms. This relative changein the population of the nonabsorbing and absorbing sublevels results ina related change in the energy absorbed from the optical radiation.Thus, the intensity of the light impinging on the photocell is afunction of the alignment of the sodium atoms.

in one embodiment of the present invention, a radio frequency magneticfield is applied to the aligned alkali atoms normal to the magneticfieldH at the Larmor frequency of the atoms in the external magnetic field Hto produce paramagnetic resonance of the atoms, that is, transitionsbetween Zeeman sublevels according to the quantum selection rule AM=J:1.At resonance, and assuming the nonabsorbing sublevels were overpopulatedrelative to the absorbing sublevels, the absorbing sublevels arepopulated at the expense of the nonabsorbing sublevels, and asubstantial increase in the optical absorption from the optical beam bythe alkali atoms takes place. The decreased radiation passing from theatom sample is detected by the optical radiation detection apparatus.Thus, the paramagnetic resonance effect may be conveniently detected bymeasuring the radiation absorbed by the atoms from the optical radiationutilized to initially optically pump the atoms.

In another embodiment of the present invention, a radio frequencymagnetic field is applied to the aligned alkali atoms parallel to themagnetic field H thereby affording field independent hyperfinetransitions. Thus, it is possible to stabilize a radio frequencygenerator to a very high degree by means of a closed feedback loop,which includes the RF generator and which is coupled to the outputcircuit of the optical radiation detection apparatus.

it is therefore an object of the present invention to provide a novelmethod and apparatus for producing and optically detecting paramagneticresonance of quantum systems exhibiting nonvanishing total angularmomentum, such as alkali elements, for example.

One feature of the present invention is the provision of a novel systemfor applying a radio frequency magnetic field to aligned to quantumsystems including, for example, alkali atoms to produce gyromagneticresonance of the atoms and for detecting said resonance by observing thevariation of the energy absorbed by the atoms from an optical radiationsource due to the realignment of the atoms during resonance, said systemproviding resonance signals with extremely narrow line widths andextremely high signal-to-noise ratios.

Another feature of the present invention is the provision of a novelsystem of the above-featured type in which the optical radiationutilized to monitor the realignment of the atoms is also utilized toinitially align the atoms by optical pumping techniques.

Still another feature of the present invention is the provision of anovel gyromagnetic resonance device for utilization in measuring unknownmagnetic fields.

Still another feature of the present invention is the provision of anovel system wherein the oscillator of a signal generator is stabilizedto a high degree resonance frequency of said gyromagnetic resonancedevice.

These and other features and advantages of the present invention willbecome apparent from a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FlG. l is a block diagram of a novel system for detecting paramagneticresonance of aligned sodium atoms by optical radiation monitoringtechniques;

FlG. 2 is a schematic diagram depicting the energy levels of the sodiumatoms of particular interest and the transitions therebetween;

FIG. 3 is an oscilloscope trace of A5 896 absorption by 3 S, sodiumatoms versus field H and shows the increase in absorption byparamagnetic resonance realignment induced by a radio frequency fieldwith the optical pumping light parallel to H and circularly polarizedclockwise;

FIG. d is a schematic drawing depicting the radio frequency transitionsinduced between the hyperfine levels of the ground state of the alkaliatoms;

HO. 5 is a block diagram of a novel alkali element system; and

FIG. 6 is a modification of FIG. 5 wherein a field independent Alt i=0hyperfine transition is used in a frequency stabilization system.

Similar numerals refer to similar elements throughout the drawing.

Referring now to FIG. 1, there is shown one embodiment of the presentinvention which utilizes an evacuated spherical glass absorption vesselll of about ll liter containing a small amount of metallic sodium inequilibrium with its vapor and containing argon at a pressure of about30 mm Hg. As pointed out in the copending application Ser. No. 313,186,this pressure is sufficiently high that the sublevel relaxation time ofatoms in the optically excited states is shorter than the time it takesthese atoms to emit radiation and return to the ground state whereby thepopulation distribution over the sublevels of the excited state isessentially randomized before emission occurs. The absorption vessel isheated by an oven 10 to such a temperature (--150 C.) that about 50percent absorption occurs. The argon acts as a buffer gas for the sodiumand it results in the realization of a relaxation time of about 0.21seconds for the ground state sublevel alignment of the sodium atoms.

The ground or lowest energy state of the sodium atoms is the 3 8,, levelwhich, due to total angular momentum considerations, is split into twohyperfine states F=l and F=2 (see FIG. 2). The vessel is located in amagnetic field H, which may for example, be the earth's magnetic field,and the F=l hyperfine state is split into three Zeeman sublevels M=0, :1while the F=2 hyperfine state is split into five Zeeman sublevels M=0,i1 :2, these Zeeman sublevels being spacedapart in the atomic spectrumby the Larmor frequency of the sodium atoms in the magnetic field H,,;in an earth's magnetic field of one-half gauss this Larmor frequency isapproximately 350 Kc/sec.

A source of optical radiation of A5896 Angstrom units is providedcomprising a commercial sodium arc lamp l2 operated from a battery 13,the lamp being mounted in a Dewar (not shown). The radiation from thislamp is focused by a condenser lens 14 through a circular lightpolarizing sheet 15 onto the absorption vessel 11. ln this particularembodiment of the invention, the optical radiation is parallel to themagnetic field H and the circular polarization is clockwise looking fromthe source 12 to the vessel 11 or right polarized. The optical radiationfrom the lamp 12, after it has pamed through the vessel 11, is focusedby lens 16 upon a vacuum photocell 17 whose output is amplified by abroad band amplifier l8 and displayed on an oscilloscope or graphicrecorder 19.

in accordance with known quantum theory selection rules, the circularlypolarized optical radiation induces AM=+l transitions of the sodiumatoms between the ground state 3 .8, and the higher energy state 3"!which are separated A5896 in the spectrum. 3 state (see FIG. 2) is splitinto two hyperfine states F=l, 2, which are in turn split into three andfive Zeeman sublevels, respectively. Due to the selection rule AM=Hconsidering the positive Z-axis pointing in the direction of the lightbeam, all of the sodium atoms in the Zeeman sublevels in the 3 8,, stateexcept those in the M=+2 sublevel of the F=2 hyperfine level absorbenergy from the A5 896 radiation and are raised to the 3 P level. Theatoms in this higher level may return to the ground state sublevels bygiving up the necessary quanta of energy by collisions or the like andquantum theory rules permit the atoms to return to the differentsublevels. As a result, the nonabsorbing M= +2 sublevel gains atoms atthe expense of other sublevels until a saturation polarization isattained.

The amount of radiation absorbed by the sodium atoms may be deten'ninedby means of the photoelectric cell 17, the DC output of the photocell 17being a direct function of the A5 896 radiation impinging thereon. Thus,increased radiation absorption in the absorption vessel 11 will resultin a decrease in the DC output from the photocell 17 which may be viewedas an increased or decreased signal, by selection of suitable electricalamplification means, on the recorder-oscilloscope device 19.

Actually, the process of optical pumping for alignment of the sodiumatoms is more complex than the simple illustration given above asexplained in the above cited patent application Ser. No. 313,186. Thereis also present in the sodium light source a A5890 radiation in additionto the A5896 (these two radiations are so-called D lines of sodium) andthis x5890 radiation is of the proper frequency to raise the sodiumatoms from the ground state to the 3 P energy state which is composed offour hyperfine states F=0, l, 2 and 3, which in turn comprise 16 Zeemansublevels. This affects the number of sodium atoms which populate thesublevels of the ground state but a preponderance of one radiation orthe other insures an alignment. Commercial sodium lamps are readilyavailable in which a substantial differential exists between theintensity of the two D lines. Also, one or the other of these D linesmay be filtered out if desired.

Other alignment processes will immediately occur to those skilled inthis art such as, for example, by a circular polarization of thetransmitted sodium light opposite, that is, counterclockwise, to that inthe above illustration, the quantum theory selection rule AM=-l governsand the M=2 magnetic sublevel of the hyperfine F=2 level of the groundstate is the nonabsorbing level and becomes overpopulated relative tothe remaining magnetic sublevels.

Any effect which tends to change the relative populations of themagnetic sublevels and produce a change in the number of atoms in theabsorbing sublevels will produce a change in the intensity of thetransmitted light. A substantial increase of the A5896 radiationabsorption by the sodium atoms may be accomplished by producing aparamagnetic resonance realignmentof the sodium atoms in the energystate 3 S, so as to cause transitions between the Zeeman sublevels.Thus, by applying, by means of a suitable signal generator 21 and aradio frequency coil 22 adjacent the absorption vessel 11, a radiofrequency magnetic field H, perpendicular to the direction of themagnetic field H,,, and of the Larmor frequency (350 kc.) of the sodiumatoms in the earths magnetic field H of approximately one-half gauss aresonance of the sodium atoms occurs wherein AM il transitions areinduced between the magnetic sublevels. The absorbing Zeeman sublevelswill now be populated at the expense of the nonabsorbing Zeeman sublevelM +2 during the resonance transitions. This increased population of theabsorbing sublevels results in a substantial weakening of the A5 896light detected by the photocell. By modulation techniques common tothose skilled in the art of gyromagnetic resonance, such as, forexample, by modulatingthe magnetic field H with an audio frequency sweepmagnetic field by use of suitable modulation coils 23 and associatedsweep generator 24, the point of maximum paramagnetic resonance may beperiodically swept through and viewed on an oscilloscope 25, thehorizontal sweep plates of which are coupled to the audio sweepgenerator 24. The decrease in transmitted radiation occurring duringresonance is depicted in the oscilloscope trace in FIG. 3. It isapparent that modulation of the frequency of the radio frequency fieldH, may be utilized to sweep through resonance rather than modulation ofthe magnetic field H,,. Thus, the paramagnetic resonance may be detectedby the expedient of monitoring the alignment of the sodium atoms by theobservation of the absorption of polarized optical radiation utilized tooptically pump the atoms initially.

A similar situation is encountered when either zero field AF=:l-, AM=O,i1 microwave transitions are induced between the hyperfine F groups by amagnetic radio frequency field of the appropriate direction andfrequency, 1772 me. for Na-or I and J are decoupled in strong magneticfields. The case of AF=1; AM=0, :l microwave transitions induced betweenthe hyperfine F groups of the sodium atoms is depicted in H0. 4. Inaccordance with quantum mechanics, the transitions between the F=2; M=0level and the F==l; M=0 fields. With the gyromagnetic resonancefrequency magnetic field of 1772 mc. parallel to the unidirectionalmagnetic field, the AM=0 transitions are induced, and when perpendicularthe AM=il transitions occur.

It should be understood that a microwave cavity coupled to the generator21 through a phase modulator may be used at suitable frequencies in lieuof the RF coil 22 in those cases where hyperfine transitions are to beobserved.

The sodium vapor resonance line has a thermal relaxation time T, ofabout 0.2 seconds and an extremely narrow line width corresponding to awidth in magnetic field units of about 10 gauss. The reasons for thisextremely narrow line width are believed to be:

a. Rare collisions of the relatively small number of sodium atomspresent in the absorption vessel with each other.

b. Prevention of frequent wall collisions by the highpressure buffer.

The number of sodium atoms in the absorption vessel at these vaporpressures of about 10" mm. Hg. is so small that it is impossible todetect the resonance by the usual resonance techniques of radiofrequency coils or cavities closely coupled to the sample. The opticaldetection of this resonance provides a detection method with anextremely high signal-tonoise ratio. in this method, the absorption of aquantum or photon of radio frequency energy in transitions of the sodiumatoms between Zeeman sublevels is manifested by the absorption from thetransmitted light beam of a photon of light energy. In the earth'smagnetic field the radio frequency photon frequency is about If) cyclesper second whereas the optical photon frequency is about 10" cycles persecond; thus there is an amplification of about 10* of energy absorbedin the resonance process. This serves to explain why enormoussignal-to-noise ratios can be obtained in this experiment even thoughthe process of monitoring the transmission of a light beam by a lightresponsive means such as a photocell is a relatively inefficientprocess-about l0 quanta/electron for the best layers-compared to thosewhich are used for monitoring radio frequency energy. The energy gainedthrough the magneto-optic amplification is so enormous that it greatlyoverrides all losses in the system.

In accordance with known quantum theory, the spectral frequency of theenergy quanta hY separating the Zeeman magnetic sublevels, termed theLarmor frequency, is a direct function of the strength of the magneticfield H producing the level splitting. Therefore, for a given atom, ifthe strength of the magnetic field H is known, the Larmor frequency maybe detennined and vice versa. In the sodium atom example given, theLarmor frequency was 350 kc. in the one-half gauss magnetic field. Theutilization of the present invention as a magnetometer device of highsensitivity and rapid response is immediately obvious.

One practical magnetometer device is shown in FIG. 5. Theabove-described paramagnetic resonance apparatus including the opticalradiation detecting apparatus is placed in an unknown magnetic field Hand the frequency of the applied radio frequency magnetic field from thegenerator 21 is adjusted until the minimum optical radiationtransmission is detected by the photocell 17, indicating maximumparamagnetic resonance which occurs at the Larmor frequency. From thisLarmor frequency, the magnetic field strength may be easily detemiinedby a frequency measuring equipment 29 coupled to the generator 21. Theoutput from the amplifier I8 is transmitted to a phase sensitivedetector 26 to which a reference signal is also transmitted from theaudio sweep circuit 24. The output of the phase sensitive detector 26 isa DC voltage, the sign of which is dependent on whether the resonance isshifted off maximum resonance on the high or low side and the magnitudeof which is dependent on the magnitude of the shift. This DC errorsignal is transmitted to a frequency tuning circuit 27 which operates toautomatically tune the generator 21 to the optimum resonance value. Asuitable strip chart recorder 28 may be utilized for recording thefrequency or error signal in terms of magnetic field strength. Whenutilizing the field independent hyperfine transitions discussed abovewith respect to FIG. 4, the system of FIG. 5 does not lend itself foruse as a magnetometer device in weak magnetic fields due to the fieldindependence; however, it does serve to stabilize to a very high degreethe oscillator of RF generator 21, the frequency of which is thehyperfine transition frequency (1772 me. for sodium). Because of thefield independence, it is preferable to modulate the radio frequency ofgenerator 2i to sweep through resonance rather than modulate the field HThe modifications required for such a system, which serves as afrequency stabilization system are shown in FIG. 6.

In FIG. 6, the radio frequency signal generator 21 generates analternating signal that is frequency modulated by means of a modulationoscillator 31 and phase modulator 32, in a wellknown manner. Themodulated signal is supplied through a coupler 37, to a cavity resonator30 which encompasses the absorption vessel 11. The cavity resonator 30may be a hollow cylinder, by way of example, with end portions havingslots therein to allow the optical pumping light from the lamp 12 toimpinge on the absorption cell ill, and the nonabsorbed light to pass tothe photocell 17. The microwave coupling loop is coupled from themodulator 32 to an end of the cylindrical cavity to provide a microwaveRF field H, substantially parallel to the polarizing field H so that thetransitions which are effectively independent of magnetic field may beinduced between hyperfine states. It shouldbe understood that the cavityneed not be cylindrical, but may be rectangular or of some otherconfiguration. Also, a radio frequency coil may be employed at suitablefrequencies instead of the cavity to provide the proper field.

In this embodiment of the invention, the signal detected by thephotocell 17, including the modulation frequency and harmonics, ispassed through the amplifier 18 to the phase detector 26. At the sametime, a reference signal derived from the modulation oscillator 31 isapplied to-the phase detector 26. [f the frequency of the RF generatoris not coincident with the resonance frequency detected by the photocell17, an error signal is produced by the detector 26. The error signal isdirected to the frequency tuner 27 which serves to maintain the signalproduced by the RF generator 21 at a precise frequency. The RF generator21 is therefore disposed in a closed feedback loop, and the oscillatorof generator 21 is stabilized to a predetermined frequency in responseto the signal detected by the photocell 17. In this manner, a highlystable frequency signal may be obtained from the RF generator forutilization in a circuit 33, for example.

It is also possible to investigate various alkali atomsspectroscopically by this paramagnetic resonance equipment havingprecisely determined magnetic fields H radio frequencies and opticaltransmission frequencies.

The above example of sodium atoms was utilized to describe thisinvention. It will be immediately recognized by those skilled in thisart that this invention is not limited to sodium atoms but applies, forexample, to the other alkali atoms such as potassium, rubudium andcesium which are quantum systems exhibiting nonvanishing total angularmomentum.

When using sodium, relatively high temperatures are needed both at thelight source and in the absorption cell (230 C. and C respectively).Also, the close spacing of 6 Angstrom units of the sodium D lines,requires fine filtering out of one of the lines. In the case of thealkali metals potassium, rubidium and cesium, the radiation lines whichexcite the P and P 2 energy levels are relatively far apart and may beseparated by interference filters. Potassium is quite satisfactory formany types of applications of this invention. The operatingcharacteristics of the potassium absorption cell appear to be optimum atabout 60 C., an ideal ambient temperature. The two spectral lines ofinterest for potassium are at 7,644 and 7,698 Angstrom units making thefiltering problem somewhat simpler than in the case of sodium. Rubidiumhas the advantage that it has a higher hyperfine constant resulting insmaller splitting in the earths magnetic field, and the two spectrallines of interest are several hundred Angstrom units apart makingfiltering extremely simple. The operating temperature for the rubidiumabsorption cell is near room temperature. Cesium has no common isotopesof spins 3/2, all having greater spins, resulting in a'lower Larmorfrequency in a given magnetic field. Lithium has a very low vaporpressure, requiring high operating temperatures, and also the twospectral lines are very close together (less than 1 Angstrom unit). Withregard to transitions between the hyperfine F groups of these alkaliatoms, the hyperfine separation is 462 me. for potassium (K 3036 me. forRb- 6835 me. for Rb" and 9193 me. for Cs.

Iclaim:

ll. Apparatus for monitoring the ground state sublevel alignment ofmagnetic moments of alkali atoms in vapor form in a unidirectionalmagnetic field comprising: means for optically irradiating said atomswith a circularly polarized radiation having a substantial propagationdirection component parallel to said magnetic field, said opticalradiation having a spectral frequency supplying quanta of energy toproduce transitions between energy levels to thereby align said atoms inthe magnetic field; optical radiation responsive means for detectingsaid optical radiation after it has irradiated and passed through saidatoms; and means for applying a radio frequency magnetic field to saidatoms at the gyromagnetic resonance frequency in said magnetic field tothereby produce gyromagnetic resonance of said magnetic moments, saidgyromagnetic resonance being evidenced as a change in the detectedoptical radiation, wherein said alkali atoms have a thermal relaxationtime in the ground state, and said alkali vapor is mixed with a buffergas to increase the relaxation time of the alkali atoms.

2. Apparatus for producing gyromagnetic resonance of atoms of alkalielements in vapor form located in a unidirectional magnetic field whichcomprises: a sample of said vapor mixed with a buffer gas so thatcollisions of said alkali atoms are minimized; means for opticallyirradiating said atoms with a radiation beam directed substantiallyparallel to said unidirectional magnetic field and circularly polarized,said optical radiation having a spectral frequency supplying quanta ofenergy to produce transitions between energy levels to thereby alignsaid atoms in said unidirectional magnetic field in their ground state;means for applying a radio frequency magnetic field to said atomssubstantially normal to said unidirectional magnetic field and of theLarmor frequency of said atoms in said magnetic field to thereby producegyromagnetic resonance transitions of said alkali atoms between Zeemansublevels in the ground state; and means for detecting said ground stategyromagnetic resonance transitions, including means for measuring theintensity of the optical radiation after passing through said alkalivapor.

3. Magnetic field measuring apparatus comprising an absorption vesselcontaining an alkali vapor mixed with a buffer gas adapted to bepositioned in a unidirectional magnetic field to be measured, meansincluding a light source producing an optical radiation directed throughsaid absorption vessel substantially parallel to said magnetic field andcircularly polarized to produce an alignment of said atoms in theirground state in said magnetic field by optical pumping; means forapplying a radio frequency magnetic field to the alkali atomssubstantially normal to said magnetic field to be measured wherebygyromagnetic resonance transitions of said alkali earth atoms may beproduced between Zeeman sublevels in said ground state; and meansresponsive to the nonabsorbed optical radiation generated by the lightsource for optically detecting the resonance of said atoms while saidradio frequency magnetic field is applied.

4. Magnetic field measuring apparatus as claimed in claim 3 includingmeans for modulating at least one of said unidirectional and radiofrequency magnetic fields to thereby sweep through gyromagneticresonance.

5. Magnetic field measuring apparatus comprising: an absorption vesselcontaining an alkali vapor mixed with a buffer gas adapted to bepositioned in a unidirectional magnetic field to be measured; meansincluding a light source producing an optical radiation directed throughsaid absorption vessel substantially parallel to said magnetic field andcircularly polarized to produce an alignement of said atoms in saidmagnetic field by optical pumping; means for applying a radio frequencymagnetic field to the alkali atoms substantially normal to said magneticfield to be measured whereby gyromagnetic resonance transitions of saidalkali atoms may be produced; optical radiation detecting means forintercepting said optical radiation after it has passed through saidabsorption vessel whereby the intensity of said optical radiation may bemeasured to give an indication of the absorption of the radiation by thealkali atoms, the gyromagnetic resonance being evidenced by a change insaid intensity; and means responsive to the frequency of said appliedradio frequency magnetic field at resonance for providing an outputwhich varies in accordance with the strength of said magnetic field.

6. Magnetic field measuring apparatus as claimed in claim 5, includingmeans for modulating at least one of said unidirectional and radiofrequency magnetic fields to thereby sweegthrou h said gyromagneticresonance. I

7. ppara us for monitoring the ground state sublevel alignment of alkaliatoms in vapor form in a unidirectional magnetic field comprising: meansfor optically irradiating said atoms with circularly polarized radiationhaving a substantial propagation direction component parallel to saidmagnetic field, said optical radiation having a spectral frequencysupplying quanta of energy to produce transitions between energy levelsfor aligning said atoms in the magnetic field; optical detection meansfor detecting the nonabsorbed optical radiation after it has passedthrough said atoms; means for applying a radio frequency magnetic fieldto said atoms substantially perpendicular to the unidirectional magneticfield at a resonance frequency in said unidirectional magnetic field;modulating means for periodically sweeping through the resonance of saidalkali atoms; a phase sensitive detector coupled to said opticaldetection means and to said modulating means providing a referencesignal to said phase sensitive detector for comparison with the detectedradiation; a frequency tuner coupled to the output circuit of said phasedetector for receiving such error signal; and means coupling saidfrequency tuner to said radio frequency magnetic field applying means tomaintain said field applying means stabilized to the resonancefrequency.

8. Apparatus for monitoring the ground state sublevel alignment ofalkali atoms in vapor form in a unidirectional magnetic fieldcomprising: means for optically irradiating said atoms with circularlypolarized radiation having a substantial propagation direction componentparallel to said magnetic field, said optical radiation having aspectral frequency supplying quanta of energy to produce transitionsbetween energy levels for aligning said atoms in the magnetic field;optical detection means for detecting the nonabsorbed optical radiationafter it has passed through said atoms; means for applying a radiofrequency magnetic field to said atoms at the gyromagnetic resonancefrequency in said unidirectional magnetic field; modulating means forperiodically sweeping through the resonance of said alkali atoms, saidmodulating means providing a reference signal; a phase sensitivedetector coupled to said optical detection means and to said modulatingmeans for developing an error signal; and means coupling said phasesensitive detector to said radio frequency magnetic field applying meansto maintain said field applying means stabilized to the resonancefrequency.

9. Apparatus for monitoring the ground state sublevel alignment ofalkali atoms in vapor form in a unidirectional magnetic fieldcomprising: means for optically irradiating said atoms with circularlypolarized radiation having a substantial propagation direction componentparallel to said magnetic field, said optical radiation having aspectral frequency supplying quanta of energy to produce transitionsbetween energy levels for aligning said atoms in the magnetic field;optical detection means for detecting the nonabsorbed optical radiationafter it has passed through said atoms; means for applying a radiofrequency magnetic field to said atoms substantially parallel to theunidirectional magnetic field; modulating means for periodicallysweeping through the resonance of said alkali atoms; a phase sensitivedetector coupled to said optical detection means and to said modulatingmeans said modulating means providing a reference signal to said phasesensitive detector for comparison with the detected radiation; and meanscoupling said phase detector to said radio frequency magnetic fieldapplying means to maintain said field applying means stabilized to theresonance frequency.

1. Apparatus for monitoring the ground state sublevel alignment ofmagnetic moments of alkali atoms in vapor form in a unidirectionAlmagnetic field comprising: means for optically irradiating said atomswith a circularly polarized radiation having a substantial propagationdirection component parallel to said magnetic field, said opticalradiation having a spectral frequency supplying quanta of energy toproduce transitions between energy levels to thereby align said atoms inthe magnetic field; optical radiation responsive means for detectingsaid optical radiation after it has irradiated and passed through saidatoms; and means for applying a radio frequency magnetic field to saidatoms at the gyromagnetic resonance frequency in said magnetic field tothereby produce gyromagnetic resonance of said magnetic moments, saidgyromagnetic resonance being evidenced as a change in the detectedoptical radiation, wherein said alkali atoms have a thermal relaxationtime in the ground state, and said alkali vapor is mixed with a buffergas to increase the relaxation time of the alkali atoms.
 2. Apparatusfor producing gyromagnetic resonance of atoms of alkali elements invapor form located in a unidirectional magnetic field which comprises: asample of said vapor mixed with a buffer gas so that collisions of saidalkali atoms are minimized; means for optically irradiating said atomswith a radiation beam directed substantially parallel to saidunidirectional magnetic field and circularly polarized, said opticalradiation having a spectral frequency supplying quanta of energy toproduce transitions between energy levels to thereby align said atoms insaid unidirectional magnetic field in their ground state; means forapplying a radio frequency magnetic field to said atoms substantiallynormal to said unidirectional magnetic field and of the Larmor frequencyof said atoms in said magnetic field to thereby produce gyromagneticresonance transitions of said alkali atoms between Zeeman sublevels inthe ground state; and means for detecting said ground state gyromagneticresonance transitions, including means for measuring the intensity ofthe optical radiation after passing through said alkali vapor. 3.Magnetic field measuring apparatus comprising an absorption vesselcontaining an alkali vapor mixed with a buffer gas adapted to bepositioned in a unidirectional magnetic field to be measured, meansincluding a light source producing an optical radiation directed throughsaid absorption vessel substantially parallel to said magnetic field andcircularly polarized to produce an alignment of said atoms in theirground state in said magnetic field by optical pumping; means forapplying a radio frequency magnetic field to the alkali atomssubstantially normal to said magnetic field to be measured wherebygyromagnetic resonance transitions of said alkali earth atoms may beproduced between Zeeman sublevels in said ground state; and meansresponsive to the nonabsorbed optical radiation generated by the lightsource for optically detecting the resonance of said atoms while saidradio frequency magnetic field is applied.
 4. Magnetic field measuringapparatus as claimed in claim 3 including means for modulating at leastone of said unidirectional and radio frequency magnetic fields tothereby sweep through gyromagnetic resonance.
 5. Magnetic fieldmeasuring apparatus comprising: an absorption vessel containing analkali vapor mixed with a buffer gas adapted to be positioned in aunidirectional magnetic field to be measured; means including a lightsource producing an optical radiation directed through said absorptionvessel substantially parallel to said magnetic field and circularlypolarized to produce an alignement of said atoms in said magnetic fieldby optical pumping; means for applying a radio frequency magnetic fieldto the alkali atoms substantially normal to said magnetic field to bemeasured whereby gyromagnetic resonance transitions of said alkali atomsmay be produced; optical radiation detecting means for intercepting saidoptical radiation after it has passed through said absorption vesselwhereby tHe intensity of said optical radiation may be measured to givean indication of the absorption of the radiation by the alkali atoms,the gyromagnetic resonance being evidenced by a change in saidintensity; and means responsive to the frequency of said applied radiofrequency magnetic field at resonance for providing an output whichvaries in accordance with the strength of said magnetic field. 6.Magnetic field measuring apparatus as claimed in claim 5, includingmeans for modulating at least one of said unidirectional and radiofrequency magnetic fields to thereby sweep through said gyromagneticresonance.
 7. Apparatus for monitoring the ground state sublevelalignment of alkali atoms in vapor form in a unidirectional magneticfield comprising: means for optically irradiating said atoms withcircularly polarized radiation having a substantial propagationdirection component parallel to said magnetic field, said opticalradiation having a spectral frequency supplying quanta of energy toproduce transitions between energy levels for aligning said atoms in themagnetic field; optical detection means for detecting the nonabsorbedoptical radiation after it has passed through said atoms; means forapplying a radio frequency magnetic field to said atoms substantiallyperpendicular to the unidirectional magnetic field at a resonancefrequency in said unidirectional magnetic field; modulating means forperiodically sweeping through the resonance of said alkali atoms; aphase sensitive detector coupled to said optical detection means and tosaid modulating means providing a reference signal to said phasesensitive detector for comparison with the detected radiation; afrequency tuner coupled to the output circuit of said phase detector forreceiving such error signal; and means coupling said frequency tuner tosaid radio frequency magnetic field applying means to maintain saidfield applying means stabilized to the resonance frequency.
 8. Apparatusfor monitoring the ground state sublevel alignment of alkali atoms invapor form in a unidirectional magnetic field comprising: means foroptically irradiating said atoms with circularly polarized radiationhaving a substantial propagation direction component parallel to saidmagnetic field, said optical radiation having a spectral frequencysupplying quanta of energy to produce transitions between energy levelsfor aligning said atoms in the magnetic field; optical detection meansfor detecting the nonabsorbed optical radiation after it has passedthrough said atoms; means for applying a radio frequency magnetic fieldto said atoms at the gyromagnetic resonance frequency in saidunidirectional magnetic field; modulating means for periodicallysweeping through the resonance of said alkali atoms, said modulatingmeans providing a reference signal; a phase sensitive detector coupledto said optical detection means and to said modulating means fordeveloping an error signal; and means coupling said phase sensitivedetector to said radio frequency magnetic field applying means tomaintain said field applying means stabilized to the resonancefrequency.
 9. Apparatus for monitoring the ground state sublevelalignment of alkali atoms in vapor form in a unidirectional magneticfield comprising: means for optically irradiating said atoms withcircularly polarized radiation having a substantial propagationdirection component parallel to said magnetic field, said opticalradiation having a spectral frequency supplying quanta of energy toproduce transitions between energy levels for aligning said atoms in themagnetic field; optical detection means for detecting the nonabsorbedoptical radiation after it has passed through said atoms; means forapplying a radio frequency magnetic field to said atoms substantiallyparallel to the unidirectional magnetic field; modulating means forperiodically sweeping through the resonance of said alkali atoms; aphase sensitive detector coupled to said optical detection means and tosaid modulaTing means said modulating means providing a reference signalto said phase sensitive detector for comparison with the detectedradiation; and means coupling said phase detector to said radiofrequency magnetic field applying means to maintain said field applyingmeans stabilized to the resonance frequency.